The present invention relates to a gas sensing apparatus adapted to be incorporated, for example, in an internal combustion engine exhaust emission control system employing a three-way catalyst so as to sense the air-fuel ratio.
Sensors for sensing the air-fuel ratio of internal combustion engines are known in the art which employ a transition metal oxide responsive to the composition of the exhaust gases so as to detect changes in the electric resistance value of the oxide.
As shown in FIG. 1 of the accompanying drawings, the electric resistance value of the transition metal oxide varies depending not only on the atmosphere of the exhaust gases, that is, whether the equivalence ratio .lambda. (.lambda.=actual air-fuel ratio/stoichiometric air-fuel ratio) is smaller than 1, i.e., rich or greater than 1, i.e., lean but also on the exhaust gas temperature. Thus there is a disadvantage that while, if the exhaust gas temperature is 800.degree. C., it is possible to sense the equivalence ratio .lambda. at a reference value corresponding to that shown by a solid line A and thereby to control the factor at around .lambda.=1 (stoichiometric air-fuel ratio), if the exhaust gas temperature is 400.degree. C., the equivalence ratio .lambda. can be sensed only on the considerably rich side compared with around .lambda.=1 as shown in FIG. 1 so that it will be made impossible to control the air-fuel ratio and consequently it will be made impossible to properly control the air-fuel ratio unless compensation for changes in the electric resistance caused by the exhaust gas temperatures or temperature compensation is provided.
To overcome the foregoing deficiencies, another type of gas sensing apparatus such as disclosed in West German Document Open For Inspection No. 2817873 has been proposed in which the sensing apparatus comprises a pair of sinters made of metal oxide having an electric resistance value which varies with both the relative atmosphere and the exhaust gas temperature, only one of the sinters having deposited thereon a gas composition oxidizing catalyst, and a change in the electric resistance value of the catalyst deposited sinter (catalytic element) is sensed while subjecting the catalytic element to temperature compensation by the other sinter having no catalyst (non-catalytic element), thereby sensing electric resistance value changes which are not affected by the exhaust gas temperatures and are dependent only on the relative atmosphere and accurately controlling the air-fuel ratio.
The air-fuel ratio control system employing the sensing apparatus of this prior art is disadvantageous in that while the air-fuel ratio can be controlled at a considerably wide range of temperatures from 360.degree. C. to over 800.degree. C. as shown by a dotted line a (marked with .DELTA.) in FIG. 2, any attempt to effect the control even at temperatures as low as about 280.degree. C. will control the air-fuel ratio to the lean side or result in an improper control. This is considered to be due to the fact that the resistance of the non-catalytic element becomes as high as over 10 megaohms and the resulting disturbance such as noise appearing on the signal line cannot be completely eliminated or prevented by an electric circuit or the like.